This invention relates to a micro relief element (MRE) and a method of preparing same.
An MRE, as referred to herein, is a 3-dimensional structure which is formed on the surface of a desired substrate and which structure is able to perform a specific function. Typically, the structure is a repetitive pattern which protrudes above the substrate to a defined height of the order of 0.1 to 1000 microns. Such an MRE can be used as an active component in micro-optic, micro-fluidic, micro-electrical and micro-mechanical devices. In particular, such an MRE can be used as a micro-optical element (MOE) and in which case the structure may be of a height in the range 0.1 to 1000 microns, more commonly in the range 0.1 to 10 microns. Where the MRE is a component in a micro-fluidic or micro-mechanical device then the structures are usually of heights in the range 10 to 1000 microns.
An MOE comprises a surface relief structure whose purpose is to induce phase changes on a light beam which is incident upon the structure such that a predetermined spatial distribution of the light results when the incident light is viewed either in reflection or transmission. MOEs also include structures in which the relief structure is embedded within a light transmissive material, hereinafter an immersed MOE, such as for example an immersed microlens.
MOEs may be used for a variety of applications, such as diffraction gratings, lenses, beam array generators, laser harmonic separators, focusing mirrors and microlens arrays.
Microlens arrays can be used for optical readers, interfaces between laser diodes and optical fibres, diffuser screens, integral photography, 3-d camera and display systems, integrated optical devices and imagebars.
Usually, an MOE is formed by exposing and developing the desired surface relief structure into a photosensitive material coated onto the supporting substrate and then transfering the surface relief structure into the substrate by plasma or chemical etching. The conventional design and fabrication of MOEs is discussed in xe2x80x9cSynthetic diffractive elements for optical interconnectsxe2x80x9d, M R Taghizadeh et al, Optical Computing and Processing, Vol 2(4), pp 221-242, 1992; xe2x80x9cTwo-dimensional array of diffractive microlenses fabricated by thin film depositionxe2x80x9d, J Jahns et al, Appl Opt, Vol 29(7), 931, 1990; xe2x80x9cContinuous-relief diffractive optical elements for two-dimensional array generationxe2x80x9d, M T Gale et al, Appl Opt, Vol 32(14), 2526, 1993; xe2x80x9cMultilevel-grating array generators: fabrication error analysis and experimentsxe2x80x9d, J M Miller et al, Appl Opt, Vol 32(14), 2519, 1993; and xe2x80x9cFabricating binary optics in infrared and visible materialsxe2x80x9d M B Stem et al, SPIE, Vol 1751, Miniature and micro-optics, pp 85-95, 1992.
Microlens arrays have in the past been produced by different methods as described in xe2x80x9cPolymer microlens arraysxe2x80x9d, P Pantelis and D J McCartney, Pure Appl. Opt., Vol 3, 103 (1994); xe2x80x9cThe manufacture of microlenses by melting photoresistxe2x80x9d, D Daley, R F Stevens, M C Hutley and N Davies, Meas. Sci. Technol., Vol 1, 759 (1990); and xe2x80x9cMicrolens array fabricated in surface relief with high numerical aperturexe2x80x9d, H W Lau, N Davies, M McCormick, SPIE Vol 1544 Miniature and Micro-optics: Fabrication and System Applications, p178 (1991). Glass microlenses have been made by chemically etching glass, moulding glass, plasma etching glass to produce a surface relief structure. Polymer microlenses have been produced by melting islands of photoresist or by direct writing photosensitive materials with a laser beam or by directly writing a suitable material with an electron beam or by plasma etching or by moulding.
Unfortunately, conventional methods of fabrication for MREs are limited in the range of substrates that can be used and in the complexity and accuracy of the relief structures that can be formed.
It is an object of the present invention to provide a facile method for producing MREs, in particular MOEs, in a variety of substrates and complexity of designs. An advantage of the present method is that a wide range of heights of surface relief can be produced using the same process. Another advantage is that small lateral features can be successfully reproduced. Additionally, the process may be used to produce large area MREs.
Accordingly in a first aspect the present invention provides a micro relief element which comprises
a) a first layer of a first substrate, the first layer having a receptive surface capable of retaining a relief forming polymer;
(b) an overlay of a desired thickness of the relief forming polymer over the receptive surface; and
(c) at least one relief feature formed from the relief forming polymer and which protrudes above the overlay.
In a second aspect the present invention provides a structure for use as at least part of a micro-optical element, which structure comprises
(a) a first layer of an optically transmissive first substrate having a first refractive index, the first layer having a receptive surface capable of retaining an optically transmissive relief forming polymer;
(b) an overlay having an optically insignificant effect, preferably having a maximum thickness of less than 1.5 xcexcm, of the relief forming polymer over the receptive surface, the relief forming polymer having a second refractive index which is the same as or different from the first refractive index; and
(c) at least one optically active relief feature formed from the relief forming polymer and which protrudes above the overlay.
In a third aspect of the present invention there is provided an immersed MOE comprising
3(a) a first layer of an optically transmissive first substrate having a first refractive index, the first layer having a receptive surface capable of retaining an optically transmissive relief forming polymer;
(b) an overlay having an optically insignificant effect, preferably having a maximum thickness of less than 1.5 xcexcm, of the relief forming polymer over the receptive surface, the relief forming optically transmissive polymer having a second refractive index which is the same as or different from the first refractive index;
(c) at least one optically active relief feature formed from the relief forming polymer and which protrudes above the overlay; and
(d) a second layer of an optically transmissive second substrate having a third refractive index which is superimposed upon the at least one optically active relief feature and wherein not all of the first, second and third refractive indices are the same.
In a fourth aspect of the present invention there is provided a method of preparing a micro relief element which comprises
a) a first layer of a first substrate, the first layer having a receptive surface capable of retaining a relief forming polymer;
(b) an overlay of a desired thickness of the relief forming polymer over the receptive surface; and
(c) at least one relief feature formed from the relief forming polymer and which protrudes above the overlay
which method comprises
(a) forming a line of contact between the receptive surface and at least one mould feature formed in a flexible dispensing layer;
(b) applying sufficient of a resin, capable of being cured to form the relief forming polymer, to substantially fill the at least one mould feature, along the line of contact;
(c) progressively contacting the receptive surface with the flexible dispensing layer such that
(1) the line of contact moves across the receptive surface;
(2) sufficient of the resin is captured by the mould feature so as to substantially fill the mould feature; and
(3) no more than a quantity of resin capable of forming the overlay passes the line of contact;
(d) curing the resin filling the at least one mould feature so as to form the at least one relief feature; and, optionally, thereafter
(e) releasing the flexible dispensing layer from the at least one relief feature.
In a fifth aspect of the present invention there is provided a method of preparing a structure for use as at least part of a micro-optical element, which structure comprises
(a) a first layer of an optically transmissive first substrate having a first refractive index, the first layer having a receptive surface capable of retaining an optically transmissive relief forming polymer;
(b) an overlay having an optically insignificant effect, preferably having a maximum thickness of less than 1.5 xcexcm, of the relief forming polymer over the receptive surface, the relief forming polymer having a second refractive index which is the same as or different from the first refractive index; and
(c) at least one optically active relief feature formed from the relief forming polymer and which protrudes above the overlay
which method comprises
(a) forming a line of contact between the receptive surface and at least one mould feature formed in a flexible dispensing layer;
(b) applying sufficient of a resin, capable of being cured to form the relief forming polymer, to substantially fill the at least one mould feature, along the line of contact;
(c) progressively contacting the receptive surface with the flexible dispensing layer such that
(1) the line of contact moves across the receptive surface;
(2) sufficient of the resin is captured by the mould feature so as to substantially fill the mould feature; and
(3) no more than a quantity of resin capable of forming the overlay passes the line of contact;
(d) curing the resin filling the at least one mould feature so as to form the at least one optically active relief feature; and, optionally, thereafter
(e) releasing the flexible dispensing layer from the at least one optically active relief feature.
An MRE of the present invention may be capable of use as an active component in a micro-optic, micro-fluidic, micro-electrical or micro-mechanical device. However, the principle use herein envisaged for an MRE of the present invention is as a micro-optical element (MOE). Reference herein to features making up an MOE according to the invention may be to features which are equally advantageous in other applications of MRE""s and references to MOE""s will be construed as referring to MRE""s accordingly.
Such an MOE may be able to perform more than one optical function, e.g. an MOE for use as a beam corrective optic for diode lasers may combine the functions of astigmatism correction, elipticity correction and beam collimation.
Moreover, the optically active relief feature in combination with the supporting first layer may be able to perform more than one optical function, for example an optically active relief feature supported on a shaped first layer, suitably of lens shape, may provide for correction of chromatic aberration.
Accordingly it will be apparent that the first layer and indeed the MRE or MOE, and the relief feature(s) may be of any desired geometry according to the desired function to be performed. For example the first layer, including an optional support substrate, may be planar, hollow or solid cylindrical, or may comprise a lens or other optical component wherein the relief feature(s) is/are suitably applied to a surface thereof. Alternatively or additionally the relief feature(s) may for example comprise one or more continuous, stepped or otherwise profiled structures such as lens, straight or angled track or lateral, annular ring, straight or curved diffraction grating, multiple faced (pyramidal), or other optical, fluidic, electrical or mechanical structure.
Additionally, the MOE may be coated with an other material in order to protect the MOE (anti-scratch coating) or to reduce reflection from the MOE (anti-reflection coating). Preferably, such coatings are multilayered coatings.
Furthermore, the MOE may function in reflection rather than transmission. This might be achieved by fabricating the MOE using a reflective first layer or by coating the surface of the MOE to enhance reflection from it.
The first layer may be supported by a suitable support substrate which may be subsequently removed from the first layer. However, it is preferred that the first layer is self-supporting or is associated with a support surface of desired geometry for a desired application. Suitably the first layer is comprised of any suitable material for the intended application which may be known in the art for example it may be a polymer film (in particular a film formed from polyester, such as PET or PEN, or an other polymer such as PVC, polyimide, PE or a known biodegradable polymer, e.g. poly(hydroxy butyrate)); a material selected for its optical transparency at certain wavelengths for example ZnSe or Germanium which are capable of operation in the infra-red region between 2 and 15 micron; silicon; high temperature resistant inorganic metal oxide or ceramic such as titania or (fused) silica, e.g. glass; or it may be a natural or synthetic paper product such as a wood pulp or synthetic card or paper.
For certain applications, for example where semiconductor components are mounted onto the MRE and from which it is desirable to dissipate heat, the first layer may be coated with a layer of diamond or similar material with a high thermal conductivity.
Additionally, the first layer may be coated with an electrically conducting layer, e.g. indium tin oxide (ITO) or gold, so that an electrical contact can be made to a semiconductor component located on the surface of the first layer.
The receptive surface of the first layer may be coated with a suitable bonding agent, e.g. where the first layer is of glass, a silane coupling agent, which serves to more firmly anchor the relief feature to the first layer.
Coating of the first layer may be achieved as a continuous layer prior to forming the optically active relief structure(s) thereon, but is advantageously achieved as a layer about the optically active relief structure(s), which may be created by replication from the flexible dispensing layer during the formation of the optically active relief structure(s).
The second layer may also be supported by a suitable, optionally releasable, substrate. The second layer may be superimposed on the at least one optically active relief feature by any suitable means, e.g. lamination. The second layer may also be provided with at least one mould feature in which is moulded an optically transmissive polymer, which may be the same as the optically transmissive relief forming polymer retained on the receptive surface, and which may be so placed that at least some of the mould features of the second layer are matched with at least some of the mould features of the first layer such that they can form a composite optical component. The selection of the relief forming polymer will be dependent on the intended use of the MRE and includes silica filled, light curable resins such as those used in dentistry and those for rapid prototyping by stereolithography, UV curable liquid crystal resins, photocationic epoxy resins and those optically transmissive resins as described below.
When optically transmissive, the relief forming polymer may be selected from those known in the art including those developed as light curable adhesives for joining optical components for example those sold under the name LUXTRAK (LUXTRAK is a tradename of Zeneca plc), those developed for polymer optical fibre fabrication and those developed for optical recording using polymer photoresists. In particular the optically transmissive relief forming polymer may be formed from a suitable resin for example halogenated and deuterated siloxanes, styrenes, imides, acrylates and methacrylates such as ethyleneglycol dimethacrylate, tetrafluoropropylmethacrylate, pentafluorophenylmethacrylate, tetrachloroethylacrylate, multifunctional derivatives of triazine and phosphazene. Resins and polymers that contain highly fluorinated aliphatic and aromatic moieties are preferred.
Preferably, the optically transmissive relief forming polymer is selected to have as near as possible equal and opposite thermal expansion and thermo-optic coefficients. The advantage of this is that increases in the optical path length (and hence phase change) due to thermal expansion of the material are compensated by decreases in its refractive index. This advantage requires that the optically active relief is restrained from expanding laterally by the effect of the substrate material. This will be the case when the overlayer is small. xe2x80x9cTemperature dependence of index of refraction of polymeric waveguidesxe2x80x9d, R Moshrefzadeh, M D Radcliffe, T C Lee and S K Mohapatra, J Lightwave Tech, vol 10 (4), 420 (1992) describes a number of polymer materials having negative thermo-optic coefficients, positive thermal expansion coefficients of the same magnitude. For example, PMMA has a thermo-optic coefficient of xe2x88x921.1xc3x9710xe2x88x924 Kxe2x88x921.
Preferably, the optically transmissive polymer has a refractive index which is matched to the first refractive index, e.g. 1.51 at 633 nm when the first layer is Bk7 borosilicate glass or 1.46 at 633 nm when the first layer is quartz.
The refractive index of the optically transmissive relief forming polymer may be modified by the inclusion of suitable additives into the polymer. In particular the refractive index of the polymer may be adjusted by adding appropriate amounts of ethylene glycol dimethacrylate which can increase the refractive index (as measured at 1.32 or 1.55 xcexcm) by an absolute value in excess of 0.02 when added at a level of 30% by weight.
Furthermore, an error in the depth of the optically active relief features (compared to the designed depth) can be corrected by increasing or decreasing the refractive index of the optically transmissive relief forming polymer by an equal fractional amount.
A further advantage of controlling the refractive index of the optically transmissive relief forming polymer is that the wavelength of operation of the MOE is shifted as a result. Hence a series of MOEs can be produced from the same flexible dispensing layer so as to obtain an MOE which operates at high efficiency at the chosen wavelength. Changing the refractive index from 1.45 to 1.55 for an MOE designed to operate at 633 nm for example would result in maximum efficiency operation at 677 nm.
The overlay of the relief forming polymer is reproducibly controlled to obtain a thickness appropriate to the function of the MRE and may, even in those instances where a minimum overlay is desired, usefully serve to planarise the receptive surface. In some instances, e.g. in micro-mechanical devices, a relatively thick and uniform overlay may be desirable for example to secure the relief forming polymer firmly to the first layer. In other instances, e.g. where the MRE is an MOE, it is desirable to minimise the thickness of the overlay such that it does not interfere significantly with the optical function of the MOE, i.e. the overlay is optically insignificant. Preferably, the optically insignificant overlay has a maximum thickness of less than 1.5 xcexcm, preferably less than 1 xcexcm, and particularly less than 0.5 xcexcm over the surface of the first substrate. The average thickness of the optically insignificant overlay is preferably less than 1 xcexcm and particularly less than 0.5 xcexcm. The variation of the thickness of the overlay, whether optically insignificant or not, across the surface is preferably less than xc2x10.75 xcexcm, particularly less than xc2x10.5 xcexcm and especially less than xc2x10.25 xcexcm. This has the particular advantage of minimising wavefront error.
The optical performance of the MOE depends on the phase difference produced between parts of the light beam which travel through different areas of the surface relief pattern. The phase difference is defined by the product of the depth of the features below the surface of the MOE and the refractive index of the material in which the MOE is produced. An advantage of having less than 1 micron of overlay between the first layer and the optically active relief is that this height is well defined. Hence the MOE functions as designed. Also important is the flatness of the intervening surface between the optically active relief features of the MOE. Improved performance results if the intervening surface is flatter than the wavelength of the light being used. With minimum overlay, the intervening surface is as flat as the first layer on which it is produced. Another advantage of minimum overlay is that it reduces optical loss of the part resulting from absorption of light by the material by minimising the total thickness of material required to define the surface relief pattern.
A very significant advantage of making polymer optically active relief features on glass or another material with a low thermal expansion coefficient is that the thermal stability of the MOE component is enhanced as a result of maintaining the pitch of such optically active relief features and by minimising the volume of that material which has a relatively high thermal expansion coefficient.
In order to facilitate the curing of the resin it is preferred to use an initiator, for example a thermal and/or photoinitiator and particularly an initiator which does not absorb light at the operating wave length of the MOE. Typically, when used, an initiator is present in the resin at a concentration from 0.1 to 3.0% by weight, and preferably from 0.5 to 2.0% by weight.
Suitable photoinitiators include 2-methyl-1-[4-(methylthio)phenyl)-2-morpholino propanone-1 (Irgacure 907), 1-hydroxy-cyclohexyl-phenyl ketone (Irgacure184), isopropylthioxanthone (Quantacure ITX), Camphorquinone/dimethylaminoethylmethacrylate. Similarly a suitable thermal initiator is tert-butylperoxy-2-ethyl hexanoate (Interox TBPEH).
As the line of contact moves across the surface of the first layer the resin is effectively pushed across the surface and flows into the at least one mould feature. The rate at which the line of contact advances across the surface will depend, amongst other things, on the characteristics of the resin. Typically, the resin has a viscosity from 0.1 to 100 poise and more typically from 10 to 100 poise.
The resin may be fully retained within a mould feature as the line of contact moves from the mould feature, in which case the resin may be cured at any convenient subsequent time. However, the resin may often show some degree of resilience in the non-cured form in which case as the line of contact moves from the mould feature the resin therein will tend to relax and exude from the mould feature. Where the relief feature is part of an MOE then this relaxation of the resin can reduce the effectiveness of the MOE. To counter the relaxation of the resin it is preferred that the resin is cured before the line of contact completely moves from it.
Conveniently and preferably therefore, the resin contains a photoinitiator which is activated by a particular wavelength of light, particularly UV light. A suitable source of light may then be used to cure the resin before the pressure applied along the line of contact is released and before the resin relaxes from the retaining feature. It is especially preferred that the flexible dispensing layer is transparent to the light used and that the light is shone through the flexible dispensing layer towards the resin. In order to focus the light substantially at the tip and thereby avoiding, for example, premature curing of the resin, the angle of incidence of the light onto the line of contact may be required to be adjusted from polymer to polymer. Alternatively, for a given angle of incidence and where the first layer is at least partially transmissive to the light, the first layer may be chosen to have a thickness such that the internal refraction of the incident light acts to focus the light at the line of contact. Additionally, where the first layer is at least partially transmissive to the light and is of a suitable thickness, a mirrored support may be positioned under the first layer thereby causing the transmitted light to be reflected back to the line of contact.
The pressure is applied along the line of contact by any suitable means. Suitably, the pressure is applied using an advancing bar or flexible blade under a compressive load which may be drawn along the surface, or using a roller under a compressive load which may thus on advancement or rotation retain the resin in the nip formed by the bar, blade or roller between the flexible dispensing layer and the surface. It is therefore preferred that the resin is cured at the nip as the line of contact progresses across the surface.
The flexible dispensing layer is preferably a polymer film into which the mould features have been embossed. Such an embossed film is preferably transparent to UV light, has high quality surface release properties and is capable of remaining dimensionally sound during the moulding process. Conveniently, such an embossed film may be formed by (a) forming a master pattern having a contoured metallised surface which conforms to the required relief structure, (b) electroforming a layer of a first metal onto the metallised surface to form a metal master, (c) releasing the metal master from the master pattern, (d) repeating the electroforming process to form a metal embossing master shim and (e) embossing the relief structure into a polymer film so as to form the desired mould features.
Adventitiously, when transparent, the embossed film may be optically aligned so that the mould features may be precisely aligned on the receptive surface of the first layer. Thus, the mould features may be more easily oriented on the receptive surface, e.g. about a desired axis of or existing feature on the receptive surface. In particular, where the first layer is itself a lens then the optical axis of the lens may be aligned with that of an optically active relief feature formed using the mould features such that the optical performance of the composite component is optimised.
Additionally, the embossed film, if retained on the receptive layer, may serve as a protective layer which can be removed at a later time.
A further advantage of making the MOE by the above method is that the refractive index of the relief forming polymer may be varied so as to improve or modify the optical performance of the MOE. This is also a benefit because optical components with different operating wavelengths can be made from the same master shim.
A further advantage of making the MOE by the above method is that the master pattern can be made by a wide range of available techniques in a wide range of materials and is not limited to being made in a material with good optical properties. For example the original master pattern can be made by direct electron beam patterning of photoresist, conventional photolithography, silicon micromachining (K E Peterson, Proc IEEE, Vol 70, 420 (1982)), laser beam writing (E C Harvey, P T Rumsby, M C Gower, S Mihailov, D Thomas, Excimer lasers for Micromachining, Proc of IEE Colloquium on Microengineering and Optics, February 1994, digest No. 1994/043, paper 1; D W Thomas et al, Laser ablation of electronic materials, European Mat Res Soc Monographs, Vol 4, Ed. E Fogarassy and S Lazare, p221 (1992); H Schmidt, Micromachining by lasers, Conf on Lasers and Electro-optics (CLEO EUROPE 94), Amsterdam, September 1994, Paper CMB1); plasma etching (D L Flanmn in Plasma etchingxe2x80x94an introduction ed by D M Manos and D L Flarnm, Academic Press Inc, London (1989), Chapter 2); and single point diamond turning.
A further advantage of making the MOE by the above method is that the flexible dispensing layer may be treated with any suitable material for any desired purpose, for example a masking or screening medium, a priming medium, or a medium conferring any desired optical, electrical, mechanical or fluid properties, such as ink, seed (catalyst) material, a metal precursor, an electrically conducting (precursor) medium, or a biological culture or the like which may be transferred by contact reproduction to the first layer or the overlayer as desired, for example to selected regions thereof on or about the relief features, using a modification of known techniques for example as described in Appl. Phys. Lett. 68(7), 1022-23
Moreover microlenses comprising relief features having a wide range of aspect ratios, i.e. of height to width ratio, may be produced, for example of aspect ratio up to 20, suitably up to 10 or up to 15 depending on the relief forming polymer and the relief feature shape.
An advantage of fabricating an MOE in the form of a microlens array by the above method is that the shape of the surface of each lens is determined by the mould and not by the fabrication process. This is in contrast with the conventional method of producing microlens arrays which relies on surface tension of a molten material to shape the microlenses. The conventional method limits the maximum radius of curvature of each lens and hence the F-number of the lenses that can be produced. The above method can be used to produce for example aspheric lens shapes which give improved lens performance (less spherical aberration).
A further advantage of fabricating a microlens array by the above method is that a second optically functional surface or diffractive optical element, for example, can be formed on the surface of each of the lenses in the array at the same time as the lens itself is defined by use of a mould having the appropriate surface profile or diffractive structure on its inner surface. Thus a profiled or combined refractive diffractive lens is produced. Such a combined lens performs a similar optical function to an achromatic doublet lens (the combination of a lens of negative dispersion with one with positive dispersion).
A further advantage of the above method is that large areas of micro relief arrays can be produced at once, in particular microlens arrays which are often required for use as display screens. Micro relief arrays may comprise repeating sections of identical or different relief features.
Due to the sub-micron resolution of the above method, microlenses with small diameters and pitches may be produced.
A further advantage of the above method is that a set of substantially identical structures may be produced. These may be used in associated or unassociated arrangement.
In optical systems which use microlens arrays there is sometimes a requirement for an optical element which consists of two identical microlens arrays placed back to back, separated by a fixed distance related to the focal length of the microlens array and with the two arrays aligned relative to one another. An advantage of the above method is that because the same mould can be used to form each array, the two arrays will be identical. Accurate separation of the two arrays can be achieved by controlling the thickness of the intervening first layer and the focal lengths of each array can be adjusted by changing the refractive index of the second array until the distance which separates the arrays is substantially the sum of their focal lengths. Furthermore, because the method can use an optically transparent flexible dispensing layer, the second microlens array can be accurately aligned on the back of the first layer by viewing through the flexible dispensing layer.
The concept and applications of fabricating arrays of light emitting diodes with integrated diffractive microlenses fabricated by a different method has recently been reported in xe2x80x9cArrays of light emitting diodes with integrated diffractive microlenses for board-to-board optical interconnect applications: design, modelling and experimental assessmentxe2x80x9d, B Dhoedt, P D Dobbelaere, J Blondelle, P V Daele, P Demeester, H Neefs, J V Campenhout, R Baets, Conference on Lasers and Electro-Optics (CLEO Europe 94), Amsterdam, August 28 to September 2, paper CTh164 (1994). The above method may also be used with a transparent embossing film to form MOEs onto the surface of a substrate which already has semiconductor devices which emit or detect light (e.g. laser diodes, light emitting diodes, photodiodes and vertical cavity lasers) such that the MOE features are accurately aligned with the semiconductor devices.
The above method may also be used to produce MREs which are alignment layers for liquid crystal cells. Some types of liquid crystal material, in particular ferroelectric liquid crystals, require alignment layers in the cell to orient the liquid crystal in a certain way. Conventionally, the alignment layer can be produced by physically patterning the glass surface, for example by rubbing the surface in the required direction. Alternatively, a thin layer of a material such as MgF2 is evaporated onto the surface. The purpose of this alignment layer is to align the liquid crystal material with a small tilt relative to the normal to the surface. By varying the angle of evaporation, the angle of the tilt can be varied. The current drawback of this method is that the surface area is limited by the size of the evaporator""s chamber. An advantage of the above process is that a larger surface area may be structured using an embossed film prepared from several master shims. Alternatively, alignment structures for liquid crystals may be made for example in the form of a plurality of high aspect ratio MRE""s resembling relief xe2x80x9chairsxe2x80x9d of the order of 200 nm high and 20 nm wide. Adventitiously, the ability to minimise the overlay is that there is less material covering the electrode which is used to apply an electric field to the liquid crystal cell thereby potentially resulting in lower switching powers.